The present invention relates to a chromakey signal producing apparatus used for making montage pictures in TV program production, and more particulary to a line chromakey producing apparatus for producing a chromakey signal directly from an NTSC composite video signal.
Recently, an apparatus for producing a chromakey signal directly from a composite video signal, rather than from red (R), green (G) and blue (B) components, has been developed. In the prior art apparatus, a chromakey signal is produced by extracting Y, I and Q components from an incoming digital composite video signal, computing the differences between these components and corresponding Y, I and Q components of a set color (reference color) which is to be keyed, and multiplying the respective differences thus computed by sensitivity factors. In other words, only one reference color, i.e., one set of Y, I and Q components is designated as a key-color in the prior art. This prior art apparatus can generate a clear chromakey signal only when the incoming digital video signal has chrominance or luminance sufficiently distinguishable from the reference color in an area except the chromakey area. However, it is difficult to generate a clear chromakey signal when the incoming digital video signal has a color similar to the reference color in an area other than the chromakey area. Consequently, in the prior art, compensation must be made by manually adjusting the sensitivity factors in accordance with the color change of the incoming video signal or the reference color. Such manual adjustment is extremely complicated and requires a skilled technician.